To the Editor.—Dr Dugan and colleagues1 propose that a prostate cancer is clinically insignificant if it has a low ageadjusted Gleason score and will grow to no more than 20 cm3 in volume by the time of expected patient death. These intriguing criteria, which interpret the risk of progression in the context of life expectancy, deserve further study. But the authors make several leaps in logic when they suggest that the formula "defines" clinically significant disease or that it proves that 85% to 100% of the prostate cancers removed at their institution are clinically significant.First, the conclusion assumes that their model for predicting tumor growth is reliable; however, the authors acknowledge that assumptions about fixed doubling times, life expectancy, and other estimates are open to question. Second, even if the model accurately predicts whether the tumor volume will exceed 20 cm3 by the time